Recently, amplification type solid-state imaging devices in which the pixels are each provided with an amplification capability and read is performed by a scan circuit and, in particular, CMOS (Complementary Metal Oxide Semiconductor) type image sensors of which the pixels, peripheral drive circuits and signal processing circuits are of CMOS type, are widely used. In the CMOS type image sensor, it is necessary to form a photoelectric conversion part, an amplification part, a pixel selection part and so on in one pixel, and several MOS transistors (hereinafter sometimes abbreviated to Tr) are normally employed besides the photoelectric conversion part constructed of a photodiode (hereinafter sometimes abbreviated to PD).
FIG. 10A shows the construction of one pixel in the case of a PD+3Tr system. The diagram shows a photodiode 1, a detection node 3 and a reset part constructed of a MOS transistor 4 and a drain 5 to which a power voltage VD is applied. An amplification part 6 constructed of a MOS transistor, a pixel selection part 7 constructed of a MOS transistor, a signal line 8, a reset clock φRS and a pixel selection clock φS are also shown therein. FIG. 10B shows the operation of FIG. 10A by potential.
In FIGS. 10A and 10B, the photodiode 1 is first reset to the potential VD by reset operation of the reset part 4, and thereafter, a signal charge generated by incident light hv in the photodiode 1 is stored in the detection node 3 in a floating state. A potential VS of the detection node 3 falls from the potential VD due to the storage of electric charge, and the quantity of fall is proportional to the intensity of incident light and a storage period. Therefore, a variation ΔVS in the. potential VS is proportional to the intensity of incident light in the case of the storage in a certain period. The value is amplified in the amplification part 6 and thereafter selected by the pixel selection part, i.e., switch 7 for read to the signal line 8. Since the signal is proportional to the intensity of incident light in the construction of FIG. 10A, saturation disadvantageously occurs with a sufficiently intense quantity of light, failing in obtaining a wide dynamic range.
Accordingly, as shown in FIGS. 11A and 11B, a system, in which a photocurrent is read by logarithmic compression in order to obtain a wide dynamic range of incident light, is proposed. FIG. 11A is a diagram of a circuit construction of one pixel of the example. Although the case of an n-channel type is described below, the same argument can be similarly applied by inverting the polarity in the case of a p-channel type. The diagram shows a photodiode 1, a detection node 3, a logarithmic compression transistor 4 and a drain 5 to which a power voltage VD is applied. An amplification part 6, a pixel selection part 7, a signal line 8, a pixel selection clock φS and the power voltage VD are also shown. What is significantly different from the case of FIG. 10A is the arrangement that the DC (Direct Current) voltage VD is applied to the gate of the transistor 4 and the logarithmic compression is performed instead of the reset operation. The operation is described below. FIG. 11B is a diagram showing the operation of the transistor 4 in FIG. 11A by the potential relation.
As shown in FIG. 11A, since the gate voltage of the transistor 4 is fixed to the DC potential VD, the potential becomes a constant value ψG (H). When the source potential VS of the transistor 4 becomes deeper than the constant value ψG (H), the transistor 4 operates to perform weak inversion operation, i.e., flow a subthreshold current Isubth. Since the source potential VS changes so that the subthreshold current Isubth becomes equal to the photocurrent Ip, the source potential VS eventually comes to have a value that is proportional to log(Ip), i.e., obtained by logarithmically converting the photocurrent. This makes it possible to achieve responses throughout a very wide range of the quantity of incident light and obtain a very wide dynamic range.
The logarithmic conversion type image sensor shown in FIGS. 11A and 11B is the device for performing detection in a steady state in which the photocurrent and the subthreshold current are balanced with each other. With a small quantity of incident light, the device cannot use the technique of increasing the amount of signal charge by increasing the storage time as in the storage type image sensor shown in FIGS. 10A and 10B. Furthermore, since a lower limit value Imin of the photocurrent that can be logarithmically converted is restricted by the dark current of the photodiode, an increase in the dark current due to a rise in temperature or the like causes a significant reduction in low-illuminance sensitivity. For the above reasons, the low-illuminance sensitivity of the logarithmic conversion type image sensor is usually inferior to that of the storage type image sensor.
Accordingly, as shown in FIGS. 12A and 12B, a system with a single device that exhibits a linear photoelectric conversion characteristic when the optical input is small and a logarithmic photoelectric conversion characteristic when the optical input is large is proposed (refer to, for example, JP H10-90058 A and JP 2000-175108 A). FIG. 12A shows the construction of one pixel including a photodiode 1, a detection node 3, a reset part 4 and a drain 5 to which a power voltage VD is applied as in FIG. 10A. An amplification part 6, a pixel selection part 7, a signal line 8 and a pixel selection clock φS are also shown. The power voltage VD and a voltage VH that is sufficiently higher than the power voltage VD are alternately applied to the gate VG of the reset part 4 via a switch 9 in a constant cycle. The operation of FIG. 12A is indicated by potentials in FIG. 12B and by timing in FIG. 12C. In FIGS. 12A, 12B and 12C, the voltage VH is first applied to the gate VG of the reset part 4 by the switch 9 in a period T2. At this time, the potential ψG(VH) under the gate of the reset part 4 becomes deeper than the power voltage VD, and the potential of the detection node 3 is reset to the power voltage VD. Next, the power voltage VD is applied to the gate VG of the reset part 4 by the switch 9 in a period T1. At this time, the potential ψG(VD) under the gate of the reset part 4 becomes shallower than the power voltage VD, and the potential of the detection node 3 enters a floating state. When a signal charge is generated by incident light hv in the photodiode 1, the signal charge is stored in the detection node 3. In accordance with the storage of the signal charge, the potential VS of the detection node 3 is reduced from the power supply voltage VD. The quantity of reduction is proportional to the intensity of incident light and the storage period. Therefore, in the storage of a certain period, a variation ΔVS1 of the potential VS of the detection node 3 is proportional to the intensity of incident light. When the potential VS of the detection node 3 is reduced to a certain voltage value ψ0, weak inversion operation occurs, i.e., a subthreshold current Isubth flows. Since the potential VS of the detection node 3 is changed by a variation ΔVS2 from the value ψ0 so that the subthreshold current Isubth becomes equal to the photocurrent Ip, and eventually, the value ΔVS2 is proportional to log(Ip). That is, a value obtained by logarithmically converting the photocurrent results.
According to the above, the variation ΔVS1 of the potential VS of the detection node 3 is proportional to the intensity of incident light when VD≧VS>ψ0, and the variation ΔVS2 of the potential VS of the detection node 3 is proportional to log(Ip) when ψ0≧VS>ψG(VD). In this case, ψG(VD) is the potential under the gate of the reset part 4 when the power voltage VD is applied to the gate VG. Therefore, a change in the potential VS of the detection node 3 with respect to the incident light exhibits a linear photoelectric conversion characteristic when the optical input is small and exhibits a logarithmic photoelectric conversion characteristic when the optical input is large as shown in FIG. 12D. As a result, it is possible to provide linear type operation of a high sensitivity at a low illuminance and logarithmic operation of a wide dynamic range at a high illuminance.
However, the system of FIGS. 12A, 12B, 12C and 12D has the following problems. First, the potential value, i.e., the boundary between the linear operation and the logarithmic operation ψ0 varies every pixel. Therefore, very large harsh fixed pattern noises are generated without modification in the logarithmic operation region. Next, since the detection node 3 (assumed to have a capacitance C1) is reset every time in the photodetection operation, so-called kTC noises (thermal noises) expressed by electron count as:Δn=(kTC1)1/2/q occur and become random noises. In the equation, k represents the Boltzman's constant, T represents the absolute temperature and q represents the amount of electronic charge. These fixed pattern noises and random noises largely deteriorate the image quality.